Cast irons are used for a wide variety of applications and industries that include government/defense, farm and heavy truck equipment, pumps, valves, and compressors. The basic types of cast iron may be categorized as: grey cast iron, where the graphite exists mainly as elongated flakes or lamellar particles; compacted graphite iron (CGI), where the graphite particles are elongated as in grey iron but are shorter and thicker and have rounded edges and irregular bumpy surfaces; malleable iron, where the graphite particles exist as compacted aggregates; and ductile iron, where the graphite particles exist as individual nodules or spheroids, and as such may be referred to as “nodular iron” or “spherulitic iron.” The production, properties and applications of these irons is described in “The Iron Castings Handbook,” Iron Castings Society (1981), C. F. Walton (Editor), which is incorporated herein by reference in its entirety. Of these irons, ductile iron has become the iron of choice for many applications because it is exhibits relatively high strength, toughness, and endurance limits. The properties of ductile iron are further described in the publication “A Design Engineer's Digest of Ductile Iron,” (available fromthe Ductile Iron Marketing Group of the Ductile Iron Society at its website), which is incorporated herein by reference in its entirety.
Typically, the composition of unalloyed ductile iron is similar to that of grey iron with respect to the concentration of commonly present elements such as carbon, silicon, manganese, and phosphorus. The nodular or spherulitic structure of alloyed ductile iron is produced by adding one or more elements to the molten metal iron to promote nodules or spheroids (e.g., magnesium), such agents commonly being referred to as “nodularizing agents.”
Ductile iron may be utilized as-cast or may be further treated. As-cast ductile iron may contain microstructure that influences the physical properties of the iron. For example, as-cast ductile iron may include pearlitic, ferritic, and/or cementitic microstructure. The relative amount of these microstructures will depend on the composition of the iron alloy and the process used for preparing the cast iron. After casting, the iron further may be treated in annealing, quenching, or tempering processes in order to alter the microstructure of the ductile iron and to obtain a finished ductile iron product having desirable physical properties (e.g., ferritic properties). However, these further treatments will add to the final cost of the finished ductile iron product. Methods for making ductile iron casting are described in U.S. Pat. Nos. 4,475,956 and 4,484,953, the contents of which are incorporated herein by reference.
Ferritic ductile iron (60-40-18) may be characterized as iron having at least about 60,000 psi tensile strength, at least about 40,000 psi yield strength, and at least about 18% elongation. In order to obtain a ductile iron with high elongation (e.g. 18% minimum), the ductile iron should have a relatively low percentage of pearlite in its microstructure. In order to minimize the amount of pearlite microstructure in the ductile iron, elements that promote pearlite microstructure should be minimized or avoided altogether, such as copper, manganese, and chromium. In addition, pearlite microstructure further can be minimized by adding elements that promote ferrite microstructure, such as silicon. However, in order to obtain ductile iron with relatively high impact properties, not only should pearlite microstructure be minimized but silicon should be kept at a level of about 1.95-2.25% by mass, because silicon is known as an element that embrittles the ferrite microstructure or shifts the brittle→ductile transition temperature for the iron alloy to higher temperatures. In other words, every iron alloy has a transition temperature where the fracture propagation system changes from brittle to ductile. As the amount of silicon in the alloy is increased, the temperature where a brittle fracture will occur is increased, causing the iron to have low impact resistance even at the higher temperature.
On the other hand, if the ductile iron contains only low levels of silicon, the iron will have relatively low strength. In addition, a ductile iron with relatively low pearlite microstructure will not have a tensile strength of at least about 60,000 psi and a yield strength of at least about 40,000 psi. In order to compensate for the reduced strength, nickel may be added to the alloy at a concentration of about 0.50-1.00% by mass. The presence of nickel in the alloy increases the tensile and yield strengths without promoting a large amount of pearlite, thereby promoting strength without compromising impact resistance. However, the relatively high cost of nickel will increase the cost of the final ductile iron product.
Therefore, it is desirable to obtain a ferritic ductile iron casting that does not require further treatment after casting (e.g., annealing) and that does not require the addition of nickel to the alloy.